An automatic transmission for a vehicle selectively supplies hydraulic fluid to frictional engaging elements such as clutches and brakes to connect a desired rotary element in its gear system to an input shaft of the transmission or fix the element to the transmission casing, thereby automatically changing the speed ratio according to operation conditions of the vehicle.
Such an automatic transmission for a vehicle is required to be small in speed-shift shocks to protect various parts and components and maintain comfortable drive feeling. For this purpose, an automatic transmission for a vehicle has been proposed which uses a proper electronic control over the hydraulic pressure and its supply timing to frictional engaging elements, aiming to achieve reduced speed-shift shocks.
As shown in FIG. 5 which shows an example of the structure of such an automatic transmission for a vehicle, a crank shaft 12 of an engine 11 is integrally connected with an impeller 14 of a torque converter 13. The torque converter 13 has the impeller 14, a turbine 15, a stator 16, and a one-way clutch 17. The stator 16 is connected to a transmission casing 18 through the one-way clutch 17. By the function of the one-way clutch, the stator 16 is allowed to rotate in the same direction as the crank shaft 12 but is not allowed to rotate in the reverse direction. The torque transmitted to the turbine 15 is transmitted to the input shaft 19 (hereinafter referred to as the "transmission input shaft") of a gear transmission apparatus to achieve four forward speeds and a single reverse speed disposed at the rear of the torque converter 13.
The gear transmission apparatus comprises three clutches 20, 21, and 22, two brakes 23 and 24, one one-way clutch 25, and one ravigneaux type planetary gear mechanism 26. The ravigneaux type planetary gear mechanism 26 comprises a ring gear 27, a long pinion gear 28, a short pinion gear 29, a front sun gear 30, a rear sun gear 31, and a carrier 32 which rotatably supports the pinion gears 28 and 29 and is rotatably engaged with the transmission input shaft 19.
The ring gear 27 is connected to a transmission output shaft 33. The front sun gear 30 is connected to the transmission input shaft 19 through a kickdown drum 34 and a front clutch 20. Furthermore, the rear sun gear 31 is connected to the transmission input shaft 19 through a rear clutch 21. The carrier 32 is connected to the transmission casing 18 through a low reverse brake 24 and the one-way clutch 25 and to the transmission input shaft 19 through a 4th-speed clutch 22 disposed at the rear end of the gear transmission apparatus. The kickdown drum 34 is integrally connectable to the transmission casing 18 by a kickdown brake 23. Torque passed through the ravigneaux type planetary gear mechanism 26 is transmitted from a drive gear 35 mounted to the transmission output shaft 33 to the drive shaft side of driving wheels (not shown).
The clutches 20 to 22 and the brakes 23 and 24 as frictional engaging elements individually comprise hydraulic mechanisms provided with engaging piston devices or servo mechanisms. These hydraulic mechanisms are operated through a hydraulic control unit (not shown) by hydraulic fluid generated by an oil pump 36 connected to the impeller 14 of the torque converter 13.
Detailed structure and functions of the mechanisms are already known, for example, in Japanese Patent Publication Laid-open 58-54270/1983, 58-46248/1983, or 61-31749/1986. Thus, selective engagement of various frictional engaging elements is achieved according to the position of a shift lever provided beside the driver's seat of the vehicle (not shown) selected by the driver and operation conditions of the vehicle, and various speed ratios are automatically achieved through the hydraulic control unit according to instructions from an electronic control unit to control the operation conditions of the engine 11.
The select pattern of the shift lever includes P (parking), R (reverse), N (neutral), D (automatic three forward speeds or automatic four forward speeds), 2 (automatic two forward speeds), and L (fixed to the 1st speed) positions. With the shift lever set to the D position, when an auxiliary switch (over-drive switch, not shown) is operated, the automatic three forward speeds or the automatic four forward speeds can be selected. The functions of the individual frictional engaging elements when the shift lever is set to the individual positions are shown in FIG. 6. In the figure, symbol ".largecircle." indicates that an engagement condition is achieved by hydraulic operation, and symbol " " indicates that the engaging is achieved only when the L position is selected.
For example, when the shift lever is shifted from the N position to the D position during a standstill condition of the vehicle, from a condition where all of the frictional engaging elements are not engaged, only the rear clutch 21 is newly engaged to achieve the speed ratio of the 1st speed. However, during a standstill condition of the vehicle and when the accelerator pedal is not pressed down, the speed ratio of the 2nd speed is achieved in which further the kickdown brake 23 is lightly engaged, thereby preventing generation of an excessive creeping.
As shown in FIG. 7 which schematically shows the structure of the main portion of the hydraulic circuit in the hydraulic pressure control unit, the rear clutch 21 is connected with a rear clutch exhaust valve 37 through an oil passage 38, and the rear clutch exhaust valve 37 is connected with a N-D control valve 39 through an oil passage 40. The N-D control valve 39 which is supplied with hydraulic oil, of which the maximum pressure is regulated by a relief valve (not shown), from the oil pump 36 through an oil passage 41, and a 1-2 shift valve 44 which connects an oil passage 43 to a kickdown servo 42 which controls the operation of the kickdown brake 23 connect through an oil passage 45, and the 1-2 shift valve 44 is connected to a shift control valve 46 through an oil passage 47. A manual valve 48 which is mechanically linked with operation of the shift lever is connected to an oil passage 49 branched from the oil passage 41 to supply a line pressure to the manual valve 49, and the shift control valve 46 and the manual valve 48 connect through an oil passage 50. An oil passage 51 branched halfway from the oil passage 50 is connected with a hydraulic pressure control valve 52, and the hydraulic pressure control valve 52 is connected also to the N-D control valve 39 through an oil passage 53. Furthermore, the manual valve 48 and the N-D control valve 39 connect through an oil passage 54 branched halfway from the oil passage 51.
The hydraulic pressure control valve 52 supplies the line pressure supplied to the oil passages 50 and 51, adjusted by a reducer valve (now shown) to a lower pressure than the line pressure and controlled to a desired pressure according to the control hydraulic fluid supplied from the oil pump 36, to the oil passage 53 through an oil passage 55. Control hydraulic pressure in the oil passage 55 is adequately discharge-controlled by an oil pressure control electromagnetic valve of a type which closes when unenergized according to an instruction from an electronic control unit (hereinafter described as ECU) 56 so that a desired pressure is obtained.
The shift control valve 46 is controlled by a pair of shift control electromagnetic valves of a type which closes when unenergized of which the combination of actuation conditions is controlled by ECU 56, so that a central spool 60 is select controlled to positions corresponding to the 1st to 4th speed ratios.
In this case, when the shift lever is the N position as shown in FIG. 7, the line pressure from the oil passages 41 and 49 is not supplied to the oil passage 50 and 51 side, the rear clutch 21 and the kickdown brake 23 are not engaged, the transmission input shaft 19 runs idle, and the driving force from the engine 11 is not transmitted to the transmission output shaft 33.
From this condition, when the driver operates the shift lever to select the D position, and when the vehicle is in a standstill condition and the accelerator pedal is not pressured down, as shown in FIG. 8 which shows the relationship between the position of the shift lever at that time, an output signal from an inhibitor switch, a duty ratio of the hydraulic pressure control electromagnetic valve 57, the line pressure and a creep pressure supplied to an engaging side oil chamber 65 of the kickdown servo 42, only the shift control electromagnetic valve 59 of the pair of shift control electromagnetic valves 58 and 59 to control the operation of the shift control valve 46 is energized, and the line pressure from the oil passages 41 and 49 is supplied to the oil passage 50 and 47 through the central spool 60 of the shift control valve 46.
This moves the spool 61 of the 1-2 shift valve 44 to the right in FIG. 7, causing the oil passages 45 and 43 to communicate with each other. Furthermore, at the same time the shift lever is turned over from the N position to the D position, the inhibitor switch is turned on to supply the line pressure from the oil passage 51 to the oil passages 54 and 45, rapidly raising the hydraulic pressure in the oil passage 43 connecting to the kickdown servo 42 in area (1), and to the rear clutch 21 through the oil passages 53, 40, 38 to remove play of the rear clutch 21, reverting it back to the condition immediately before the engagement. In this case, since the rear clutch 21 is pressed by the amount of play, the line pressure in the oil passage 40 is not raised and, in turn, the line pressure in an oil passage 62 is not raised, a spool 63 of the N-D control valve 39 is positioned at the left end, as shown in FIG. 7.
After that, in area (2), the hydraulic pressure control electromagnetic valve 57 is actuated by the duty control, a spool 64 of the hydraulic pressure control valve 52 is intermittently moved to the left side in FIG. 7, causing the oil passage 53 to communicate with an oil discharge port EX of the hydraulic pressure control valve 52. As a result, a hydraulic pressure (hereinafter called the creep pressure), adjusted to a lower pressure than the line pressure from the oil passage 51, is supplied from the oil passage 53 to the N-D control valve 39, and from the oil passage 40 via the rear clutch exhaust valve 37 and the oil passage 38 to the rear clutch 21. This causes the rear clutch 21 to mildly engage, thereby reducing shocks associated with the engagement.
In area (3), the duty ratio of the hydraulic pressure control electromagnetic valve 57 decreases to 0%, and the line pressure from the oil passage 51 is, as is, supplied to the rear clutch 21 via the oil passage 38, achieving a complete engagement of the rear clutch 21. At the same time, the line pressure from the oil passage 51 passes through the oil passage 62 and acts on the left end in FIG. 7 of the spool 63 of the N-D control valve 39. The spool 63 of the N-D control valve 39 moves to the right in FIG. 7 to act on the left end, the oil passage 54 and the oil passage 40 communicate with each other through the oil passage 62, and the oil passage 53 and the oil passage 45 communicate with each other.
After that, in area (4), the duty ratio of the hydraulic pressure control electromagnetic valve 57 temporarily increases to 100%, and the line pressure, which acted into an engaging side oil chamber 65 of the kickdown servo 42, is discharged from an oil discharge port EX of the hydraulic pressure control valve 52 through the oil passage 43, the 1-2 shift valve 44, the oil passage 45, the N-D control valve 39, and the oil passage 53.
Then, in area (5), the hydraulic pressure control electromagnetic valve 57 becomes conductive at a predetermined duty ratio, the creep pressure is supplied from the oil passage 53 to the N-D control valve 39, and the creep pressure is supplied from the oil passage 45 through the 1-2 shift valve 44 and the oil passage 43 to the engaging side oil chamber 65 of the kickdown servo 42. As a result, the kickdown brake 23 moderately engages to achieve the 2nd speed ratio, thereby suppressing generation of an excessive creeping.
In this condition, when the accelerator pedal is pressed down by the driver, both of the pair of shift control electromagnetic valves 58 and 59 become conductive to achieve the 1st speed ratio, the central spool 60 of the shift control valve 46 moves to the right end in FIG. 7 to close the oil passages 50 and 47, the spool 61 of the 1-2 shift valve 44 is pushed back to the left end in FIG. 7, the oil passage 43 becomes communicating with the oil discharge port EX of the 1-2 shift valve 44. This causes hydraulic fluid in the engaging side oil chamber 65 of the kickdown servo 42 to be rapidly discharged through the oil passage 43 from the oil discharge port EX of the 1-2 shift valve 44, and the kickdown brake 23 is released, immediately achieving the 1st speed ratio which engages with only the rear clutch 21.
In a prior art automatic transmission shown in FIGS. 5 to 7, in which a plurality of speed ratios are achieved by electronically controlling individual hydraulic pressures supplied to a plurality of frictional engaging elements to selectively engage these frictional engaging elements, when the shift lever is moved from the N position to the D position under a standstill condition of the vehicle, the position of the shift lever and the output signal of an inhibitor switch (not shown) are synchronized with the moving timing of a spool 66 of the manual valve 48, thereby achieving a smooth shift operation.
However, when the driver moves the shift lever from the N position to the D position very slowly, the output signal of the inhibitor switch changes over before the manual valve 48 moves from the N position to the D position to achieve complete communication between the oil passages 49 and 51, since the hydraulic pressure control unit duty controls the hydraulic control magnetic valve 57 according to the signal from the inhibitor switch, the creep pressure is supplied to the rear clutch 21 side before the spool 63 of the N-D control valve 39 does not completely move to the right in FIG. 7, and the rear clutch 21 tends to fail complete engagement.
Under such a condition, even if the driver presses down the accelerator pedal in order to start the vehicle, the rear clutch 21 tends to slip resulting in a difficulty in starting the vehicle, or deteriorating acceleration of the vehicle.
With a view to eliminate the above prior art problems, it is a primary object of the present invention to provide a shift control apparatus for a vehicle automatic transmission in which individual hydraulic pressures supplied to a plurality of frictional engaging elements are electronically controlled to achieve a plurality of speed ratios, which enables positive starting of the vehicle even when the driver slowly moves the shift lever from the N position to the D position to start the vehicle.